Positive and negative are really just arbitrary names. You can only really tell if charges are like or unlike.
Many people think positive charge means the charge is greater in some way than negative charge, just like +5 is greater than -5. But in truth these names are historical throwbacks to a time when we understood the nature of charge far less clearly than we do today!
When I read your question, it sounded to me like you wanted to know how we would identify an isolated charge as positive or negative. In truth, we could not. Any test we run would only identify a charge as being like or unlike to a second charge. (But this merely shifts the problem - how do we know what the other charge is?)
If I have misinterpreted your question, then please ignore my ramblings here! I just wondered if you wanted to know if positive charge had some defining quality that would identify it, (like is the case for black versus white light). It does not.